Examples of a method for recovering oil sand from an underground oil layer in, for example, Canada include an open-pit mining method and a steam injection method, in which high-temperature high-pressure steam is charged into an oil layer through steel pipes. Since there are only a small number of regions in which the open-pit mining can be used, the steam injection method is used in many areas.
The temperature of steam which is charged into an oil layer in the steam injection method is in a temperature range of 300° C. to 400° C. (hereinafter, referred to as “a mid-temperature range”). In the steam injection method, steam having a temperature in the mid-temperature range is charged into an oil layer under high pressure. In order to charge steam, steel pipes are used as described above.
Nowadays, in order to increase the recovery rate of heavy oil and in order to decrease laying costs in response to an increase in demand for energy, there is a demand for an increase in the diameter and strength of the steel pipe described above.
Examples of a conventional technique regarding a steel pipe for steam transportation which can be used for a steam injection method are described in Patent Literature 1 and Patent Literature 2. In Patent Literature 1 and Patent Literature 2, seamless steel pipes having a strength equivalent to API grade X80 are described, and the maximum outer diameter of such seamless steel pipes is 16 inches.
It is more difficult to increase the diameter of a seamless steel pipe than other kinds of steel pipes. In addition, in the case of a seamless steel pipe, it is necessary that large amounts of alloy chemical elements be added in order to achieve a strength of API grade X80 or higher.
Here, Patent Literature 3 and Patent Literature 4 describe techniques for manufacturing a welded high-strength steel pipe with which it is possible to increase the diameter of a steel pipe. More specifically, Patent Literature 3 and Patent Literature 4 relate to techniques for manufacturing a high-strength steel pipe having a strength of API grade X80 or higher which is manufactured by using a TMCP (thermo-mechanical control process).